Hi Arlien,
I just want to make sure you know Planet Hollywood, Rainforest Cafe, T Rex Cafe, and almost all of the other restaurants at Downtown Disney are part of the 2012 Disney Dining Plan. You can download the brochure that lists all the
Dining Plan restaurants. The Dining Plan can be added to many room reservations up to three days prior to your trip, so you still have a few more days to change your mind about this. Call Disney directly at 407-W-DISNEY if you're interested.
As for budgeting for guests not using the Dining Plan, that's extremely difficult to do because people's eating habits vary so widely. A table service entree might range from about $10 to about $50 depending on what you order and where you dine. Light eaters will cost less than big eaters. Vegetarians will cost less than carnivores. Entree only folks will cost less than those that order appetizers and desserts, etc.
My suggestion is for you to head over to the independent website allears.net. They have the complete menus, with pricing, for all the restaurants at Walt Disney World. You can use these to map out a few days of sample menus for your family. This will give you the most accurate idea of what you'll need to budget.
Good luck with your planning. Please let us know if you have additional questions.